NEUTRAL: See Research Design and Implementation Criteria Checklist below.

Research Purpose:

To investigate the association of dietary glycemic index and glycemic load with prostate cancer risk in a case-control study conducted in Italy.

Inclusion Criteria:

Men aged 46 - 74 years

Cases were men admitted for incident, histologically confirmed prostate cancer

Controls were men admitted for acute, non-malignant conditions unrelated to long-term modifications of diet

Admitted to the major teaching and general hospitals in the greater Milan area, the provinces of Pordenone and Gorizia in northern Italy, the province of Latina in central Italy and the urban area of Naples in southern Italy

Exclusion Criteria:

Excluded from the analysis were 90 cases and 99 controls with diabetes.

Description of Study Protocol:

Recruitment

Cases and controls were recruited between 1991 and 2002 in the network of major teaching and general hospitals in 4 Italian areas.

The same questionnaire was used in all study centers and it was satisfactorily reproducible and valid.

The questionnaire was administered by centrally trained interviewers during the subjects' hospital stay and included information on sociodemographic characteristics and lifestyle habits, occupational and leisure-time physical activity at various ages, problem-oriented medical history, history of cancer in first-degree relatives and anthropometric measures

Dependent Variables

Prostate cancer diagnosis

Independent Variables

Dietary glycemic index and glycemic load

Interviewer-administered 78-item food frequency questionnaire was utilized to assess the usual diet for the 2 years preceding diagnosis for cases or hospital admission for controls

Average daily glycemic index and average glycemic load of a subject's diet were computed

Control Variables

Age

Study center

Education

Family history of prostate cancer

Smoking

BMI

Physical activity

Alcohol consumption

Intake of energy, fiber and lycopenes

Description of Actual Data Sample:

Initial N: 1,204 male cases, 1,352 male controls

Attrition (final N): as above

Age: median age cases = 66 years, controls = 63 years

Ethnicity: not mentioned

Other relevant demographics:

Anthropometrics

Location: Italy

Summary of Results:

OR and 95% CI of Prostate Cancer According to Glycemic Index and Glycemic Load

Variables

Q1

Q2

Q3

Q4

Q5

Χ2 trend

Continuous OR (95% CI)

Glycemic Index

Lower cut-off

---

72.01

75.21

77.98

80.92

---

---

Cases/controls

216/296

235/277

250/262

258/251

245/266

---

---

OR (95% CI)2

1 (reference)

1.27 (0.98 - 1.65)

1.39 (1.07 - 1.79)

1.59 (1.23 - 2.06)

1.46 (1.12 - 1.89)

10.83, P < 0.01

1.28 (1.12 - 1.46)

OR (95% CI)4

1 (reference)

1.23 (0.95 - 1.60)

1.24 (0.96 - 1.61)

1.47 (1.12 - 1.91)

1.57 (1.19 - 2.07)

11.62, P < 0.01

1.28 (1.11 - 1.47)

Glycemic Load

Lower cut-off

---

168.04

203.32

240.95

291.14

---

---

Cases/controls

210/301

237/274

2377/274

266/245

254/258

---

---

OR (95% CI)2

1 (reference)

1.26 (0.97 - 1.63)

1.36 (1.05 - 1.76)

1.80 (1.39 - 2.33)

1.65 (1.27 - 2.14)

20.45, P < 0.01

1.32 (1.16 - 1.49)

OR (95% CI)4

1 (reference)

0.91 (0.69 - 1.18)

1.00 (0.75 - 1.31)

1.20 (0.90 - 1.59)

1.41 (1.04 - 1.89)

7.86, P < 0.01

1.22 (1.05 - 1.41)

2ORs were adjusted for quinquennia of age, study center, education and family history of prostate cancer

4ORs were adjusted for quiinquennia of age, study center, education, family history of prostate cancer, energy intake, alcohol consumption, smoking habit, BMI, occupational physical activity at 50 years, intake of fiber and intake of lycopene

Other Findings

Cases reported more frequently than controls a high level of education (20.4% vs 13.5%), a low level of physical activity at 50 years of age (37.9% vs 29.9%) and a family history of prostate cancer (7.1% vs 2.0%), whereas similar frequency distributions were observed for BMI, alcohol consumption and intake of lycopene.

Compared to the lowest quintile of glycemic index, the odds ratios were 1.23, 1.24, 1.47 and 1.57 for subsequent levels of glycemic index.

The corresponding values for glycemic load were 0.91, 1.00, 1.20 and 1.41.

There were significant positive trends for both glycemic index and glycemic load (P < 0.01).

No heterogeneity was found among strata of selected covariates.

Author Conclusion:

We found direct relations between dietary glycemic index and glycemic load and prostate cancer risk. Correcting for potential confounding factors did not substantially modify these associations.

Reviewer Comments:

Large numbers of cases and controls. Cases and controls differed at baseline in terms of age (median, not mean reported), education, physical activity, and family history of prostate cancer, quinquennia were controlled for in the analysis. Data based on self-report. Authors note the following limitations:

Information was collected after diagnosis, thus it is possible that the early symptoms of the disease may have caused changes in the diet

Glycemic index and glycemic load values may have some variability according to specific foods and cooking methods

Research Design and Implementation Criteria Checklist: Primary Research

Relevance Questions

1.

Would implementing the studied intervention or procedure (if found successful) result in improved outcomes for the patients/clients/population group? (Not Applicable for some epidemiological studies)

N/A

2.

Did the authors study an outcome (dependent variable) or topic that the patients/clients/population group would care about?

Yes

3.

Is the focus of the intervention or procedure (independent variable) or topic of study a common issue of concern to nutrition or dietetics practice?

Yes

4.

Is the intervention or procedure feasible? (NA for some epidemiological studies)

N/A

Validity Questions

1.

Was the research question clearly stated?

Yes

1.1.

Was (were) the specific intervention(s) or procedure(s) [independent variable(s)] identified?

Yes

1.2.

Was (were) the outcome(s) [dependent variable(s)] clearly indicated?

Yes

1.3.

Were the target population and setting specified?

Yes

2.

Was the selection of study subjects/patients free from bias?

Yes

2.1.

Were inclusion/exclusion criteria specified (e.g., risk, point in disease progression, diagnostic or prognosis criteria), and with sufficient detail and without omitting criteria critical to the study?

Yes

2.2.

Were criteria applied equally to all study groups?

Yes

2.3.

Were health, demographics, and other characteristics of subjects described?

Yes

2.4.

Were the subjects/patients a representative sample of the relevant population?

Yes

3.

Were study groups comparable?

Yes

3.1.

Was the method of assigning subjects/patients to groups described and unbiased? (Method of randomization identified if RCT)

Yes

3.2.

Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline?

If cohort study or cross-sectional study, were groups comparable on important confounding factors and/or were preexisting differences accounted for by using appropriate adjustments in statistical analysis?

N/A

3.5.

If case control or cross-sectional study, were potential confounding factors comparable for cases and controls? (If case series or trial with subjects serving as own control, this criterion is not applicable. Criterion may not be applicable in some cross-sectional studies.)

Yes

3.6.

If diagnostic test, was there an independent blind comparison with an appropriate reference standard (e.g., "gold standard")?

N/A

4.

Was method of handling withdrawals described?

Yes

4.1.

Were follow-up methods described and the same for all groups?

N/A

4.2.

Was the number, characteristics of withdrawals (i.e., dropouts, lost to follow up, attrition rate) and/or response rate (cross-sectional studies) described for each group? (Follow up goal for a strong study is 80%.)

N/A

4.3.

Were all enrolled subjects/patients (in the original sample) accounted for?

Yes

4.4.

Were reasons for withdrawals similar across groups?

N/A

4.5.

If diagnostic test, was decision to perform reference test not dependent on results of test under study?

N/A

5.

Was blinding used to prevent introduction of bias?

Yes

5.1.

In intervention study, were subjects, clinicians/practitioners, and investigators blinded to treatment group, as appropriate?

N/A

5.2.

Were data collectors blinded for outcomes assessment? (If outcome is measured using an objective test, such as a lab value, this criterion is assumed to be met.)

N/A

5.3.

In cohort study or cross-sectional study, were measurements of outcomes and risk factors blinded?

N/A

5.4.

In case control study, was case definition explicit and case ascertainment not influenced by exposure status?

Yes

5.5.

In diagnostic study, were test results blinded to patient history and other test results?

N/A

6.

Were intervention/therapeutic regimens/exposure factor or procedure and any comparison(s) described in detail? Were intervening factors described?

Yes

6.1.

In RCT or other intervention trial, were protocols described for all regimens studied?

N/A

6.2.

In observational study, were interventions, study settings, and clinicians/provider described?

Yes

6.3.

Was the intensity and duration of the intervention or exposure factor sufficient to produce a meaningful effect?

N/A

6.4.

Was the amount of exposure and, if relevant, subject/patient compliance measured?